In a typical communications network, a wireless terminal(s) communicates via a Radio Access Network (RAN) to one or more Core Networks (CN). The wireless terminal is also known as mobile station and/or User Equipment (UE), such as mobile telephones, cellular telephones, smart phones, tablet computers and laptops with wireless capability. The user equipments may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices which communicate voice and/or data via the RAN. In the following, the term user equipment is used when referring to the wireless terminal.
The RAN covers a geographical area via cells that each cell is being served by a base station, e.g. a Radio Base Station (RBS), which in some networks is also called NodeB, B node, evolved Node B (eNB) or Base Transceiver Station (BTS). In the following, the term base station is used when referring to any of the above examples. A cell is a logical entity to which has been assigned a set of logical resources such as radio channels that provides radio communication in a geographical area. The base station at a base station site physically realizes the logical cell resources such as transmitting the channels. From a user equipment perspective the network is represented by a number of cells.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a RAN using WCDMA for user equipments. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based RAN technologies.
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the base stations are connected directly to a CN rather than to RNCs. In general, in LTE the functions of a RNC are performed by the base station. As such, the RAN of an LTE system has an essentially “flat” architecture comprising base stations without reporting to RNCs.
Precise timing is important in communications networks. The network time is available when it is represented by a clock. However, not every clock is exact. The deviation of the clock needs to be checked, and the clock needs to be corrected. Communication between a plurality of clocks in the network is necessary for this. To synchronise individual clocks the more inaccurate clock is set to the more accurate one. This may also be called offset correction or error correction. Furthermore, clocks may not necessarily run at exactly the same speed. Therefore, the speed of the more inaccurate clock has to be regulated constantly. This may also be referred to as drift correction.
The Network Time Protocol (NTP) and the Simple Network Time Protocol (SNTP) derived from it are protocols for providing timing in communications networks. NTP and SNTP allow accuracies into the millisecond range.
Another method for synchronization of clocks is the use of radio signals from Global Positioning System (GPS) satellites. However, this requires relatively expensive GPS receivers in every clock as well as the appropriate antennae. This type of clock has high precision.
The Institute of Electrical and Electronics Engineers (IEEE) 1588 is a standard which relates to synchronizing of real-time clocks in the nodes of a networked system. The IEEE 1588 describes a Precision Timing Protocol (PTP) which specifies methods to distribute high accuracy time synchronization in packet networks. PTP provides accuracy in the sub-microsecond range, is easy to implement and involves low cost equipment. PTP is designed to operate in packet based networks that supports multicast communication.
Five different message types are defined for PTP:                Sync        Delay_Req        Follow_Up        Delay_Resp        Management        
Sync and Delay_Req are also referred to as event messages, because they are used as timing events by the PTP protocol. Sending and receipt time stamps are generated for the sync and Delay_Req messages. The other three messages, Follow_Up, Delay_Resp and Management are also referred to as general messages. Follow_Up and Delay_Resp are used to transmit timing information. No time stamps are generated when the Follow_Up and Delay_Resp messages are sent or received. The different messages will be described in more detail below.
Clocks in a communications network implementing PTP are organized in a master-slave hierarchy. Each slave clock synchronizes to its master clock. In general, a clock comprises at least one port which is an interface for transmitting and receiving e.g. the above mentioned messages.
Within a device in the communications network, ports may be connected to master clocks, slave clocks or they may be Transparent Clocks (TC). A transparent clock is a method specified in IEEE 1588 where the PTP protocol is transparently conveyed through a device by bookkeeping of the residence time. The transparent clock in a PTP network updates the time-interval field that is part of the PTP event message. This update compensates for switch delay and has a resolution of one picosecond. Master clocks transmit announcement messages comprising information on its capabilities. Slave clocks listen to announcements and select a preferred master clock using a “Best Master Selection Algorithm”. The slave clock then starts to listen to synchronization messages (Sync) sent by the selected master clock. The term Ordinary Clock (OC) is used to denote a clock that is located at either termination side of the PTP protocol. i.e. both the master clock or slave clock may be referred to as ordinary clocks.
FIG. 1 illustrates an embodiment of timing diagram for synchronization messages in the communications network.
Step 101
The master clock transmits the synchronization message to the slave clock. The master clock time stamps the synchronization messages with its local clock when the message is transmitted, t1.
A time stamp is a sequence of characters, denoting the date and/or time at which a certain event occurred, e.g. transmission of the synchronization message. A time stamp is the time at which an event is recorded by a computer, not the time of the event itself. In many cases, the difference may be inconsequential: the time at which an event is recorded by a time stamp, e.g. entered into a log file, should be very, very close to the time of the occurrence of the event recorded.
This data is usually presented in a consistent format, allowing for easy comparison of two different records and tracking progress over time; the practice of recording time stamps in a consistent manner along with the actual data is called time stamping.
The slave clock receives the synchronization message and records the time of reception of the synchronization message using its local clock, t2.
Step 102
The master clock transmits a Follow_Up message comprising the time stamp t1.
Step 103
In order to be able to perform synchronization, the delay of the connection between the master clock and the slave clock must be known. The slave clock may therefore initiate a delay measurement by transmitting the Delay Request message. The slave clock records the time of transmission of the Delay Request message with its local clock, t3.
The master clock receives the Delay Request message and records the time reception with its local clock, t4.
Step 104
The master clocks then forwards the time of reception, t4, to the slave clock in a Delay Response message.
Using time stamp information collected in the procedure described above, the slave clock may calculate the error or offset between its local clock and the master clock compensated for the connection delay using a simple algebraic equation.
By repeating the above procedure continuously, the slave clock will stay time locked to the master clock. It is also possible to extend this scheme to frequency locking. After initial time synchronization is performed, subsequent time offsets are taken as phase error inputs to a Phase Locked Loop (PLL) controlling the rate at which the slave clock is incrementing.
As long as the connection between the master clock and the slave clock has constant and symmetric delay and rate, very high precision timing distribution may be achieved in the network. With proper hardware support for time stamping, clock distribution accuracy in the nanosecond range is within reach.
As soon as the connection between the slave clock and the master clock is something other than a wire or a fiber, as e.g. a switch, performance is quickly deteriorated due to Packet Delay Variation (PDV) emerging from the varying time, residence time, packets spend in the device. PDV is defined as the difference between the maximum and minimum transport delay for a packet between two relevant reference points in a network.
A communications network may comprise Boundary Clocks (BC). Boundary clocks are often present wherever there is a change of the communication technology, network elements blocking the propagation of the PTP messages or network devices that inserts significant delay fluctuation in the network. A boundary clock may have more than two ports. One of the ports serves as a slave port to an upstream master clock, and the other port serves as master clock to downstream slave clocks. A boundary clock may also be described as a method specified in IEEE 1588 v2 where the PTP protocol is terminated on a slave port in a device and regenerated on one or more master port(s)
The PTP also specifies a Transparent Clock profile for network devices that implements neither a slave clock nor a master clock. Each event message, i.e. messages that are time stamped as e.g. Synchronization messages, also comprises a correction field. A transparent clock simply uses its local clock to keep track of a residence time of a PTP packet in the TC and then accumulates this time to the correction field. The residence time may be defined as the delay incurred by a data packet passing through the device. Every device that receives an event message is then able to subtract the accumulated residence times in the correction field from its local time stamp before performing calculations. By using Transparent Clocking, the impact from PDV, of deterministic origin, is reduced by at least four orders of magnitude even if the network device involved uses modest 100 ppm accuracy clocks.
Microwave transmission refers to the technology of transmitting information or power by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeters; these are called microwaves. The part of the radio spectrum comprising microwaves ranges across frequencies of 1.0 GHz-300 GHz. Microwave communications is primarily limited to line of sight propagation. A microwave radio link uses a beam of radio waves in the microwave frequency range to transmit e.g. video, audio, or data between two locations. The connection between the two link endpoints is referred to as a channel. A plurality of microwave links may be aggregated to form a composite link in order to reach a higher data capacity than can be attained in the channel bandwidth available to a single link. Design of microwave radio links always aims for efficient use of the radio spectrum. Several techniques are used to accomplish this, especially in systems optimized for packet data transport. Examples are:                Adaptive Coding and Modulation (ACM) that adjusts error correction overhead and modulation scheme to the current channel conditions.        Utilization of orthogonal properties of the radio channel like Multiple Input Multiple Output (MIMO) and polarization thus creating multiple channels at the same frequency.        Compression of headers and payload.        Application of signal techniques such as diversity reception and channel equalization to counteract adverse channel conditions.        Aggregation of multiple radio links to a logical traffic channel (bonding) each.        
These techniques result in a channel capacity that has both fast and slow variation over time. This in turn leads to a varying and asymmetric PDV that deteriorates time synchronization performance.
ACM mentioned above is a method where coding overhead and modulation scheme automatically adapts to what is currently possible over the provided physical channel.
MIMO is mentioned in the examples above and is a technique to increase throughput by utilization of some orthogonal characteristic of the radio channel. Usually in line of sight Microwave Radio Links, MIMO refers to configurations exploiting spatial orthogonallity.
Further, data processing procedures like fragmentation, error correction coding, scrambling that are commonly applied in the radio interface make it very hard to identify and time stamp the PTP event message at the physical radio interface.
For these reasons, solutions that implement either Boundary Clocks or Transparent Clocks in Microwave Radio Links, or other media converters with similar properties, tend to suffer from either bad accuracy due to high PDV or excessive overhead resulting in inefficient spectrum utilization.
FIG. 2 shows a problem with PDV introduced between a Packet Sub System 201 and—as an example—three Physical Interface blocks 205 with constant delay in a device. Three physical interface blocks 205 are shown as an example in FIG. 2, but any other suitable number of physical interface blocks 205 is applicable. The PDV is introduced due to variable rate on the physical interface and serialization delay. The packet sub system 201 comprises a clock 207, such as an e.g. boundary clock or transparent clock. The packet sub system 201 comprises first and second ports 210. Delay through a Segmentation/Bonding block 215 is not possible to accurately predict since it depends on the momentary rate of the individual physical interfaces 205, and thus finally the radio channel conditions. The physical interface 205 corresponds to a lower layer. The upper layer is over the whole system (not shown in FIG. 2).
FIG. 3 shows a problem with a Transparent Clock acting on segmented data where a time bridge over protocol layers increases complexity and overhead in a device. A packet sub system 301 comprises a clock 307, such as a boundary clock or transparent clock. The packet sub system 301 comprises first and second ports 310. The segmentation/bonding block 315 comprises a transparent clock 320 and a third port 330. Three physical interfaces 305 are connected to the segmentation/bonding block 215. The segment may comprise several packets and since the residence time must be tracked for each packet, a segment may have to comprise several correction fields. Also time stamping has to be performed over protocol layers, i.e. packet⇄segment. This either creates restrictions on how packets may be mapped to segments in order to keep PTP correction fields accessible in the segment interfaces or makes it necessary to add explicit data for time stamps on the segment protocol layer. In both cases complexity and overhead will increase. Excessive overhead adds complexity and diminishes link utilization.